Russia's lower house of parliament has unanimously approved legislation that would authorize the government to designate media outlets receiving funding from abroad as "foreign agents."

Within hours of the measure's passage, the Russian Justice Ministry sent warnings to at least five Radio Free Europe/Radio Liberty (RFE/RL) news services. The letters did not specify what potential restrictions they could face, but lawmakers have said designated media could be subjected to detailed financial reporting requirements and required to label published material as coming from a foreign agent.

The State Duma approved the amendments -- which Amnesty International said would deal a "serious blow" to media freedom in Russia -- in the third and final reading on November 15.

The move came two days after the Russian state-funded television channel RT registered in the United States under a decades-old law known as the Foreign Agents Registration Act (FARA).

RT officials had complained repeatedly about being forced to comply with the law, and warned of retaliation.

RFE/RL was one of several media outlets that Russian officials warned could be labeled a foreign agent, a list that included CNN, Voice of America, and Germany's international broadcaster, Deutsche Welle.

Earlier on November 15, Duma speaker Vyacheslav Volodin described the legislation as a "symmetrical response" to what he said was U.S. pressure against Russian journalists. And deputy Duma speaker Pyotr Tolstoi said it "will in no way affect freedom of speech in Russia."

Duma speaker Vyacheslav Volodin described it as a "symmetrical response."

But Amnesty International said the authorities would "tighten their stranglehold" if they approve the measure, worsening "what was already a fairly desperate situation for press freedom in Russia."

Russia's upper house was expected to take up the measure on November 22, after which it heads to President Vladimir Putin for his signature.

Kremlin spokesman Dmitry Peskov refused to say whether Putin would sign the measure, but he added that the bill gives Moscow the ability to respond to any "restrictions to the freedom of Russian media abroad."

Under the legislation, articles and broadcasts by registered media must be accompanied by a disclaimer informing audiences of the outlet's status as a "foreign agent."

"The measure will in no way affect freedom of speech in Russia," deputy speaker Pyotr Tolstoi said immediately before the vote.

It is unclear if the Justice Ministry will be able to shut down foreign-funded media outlets that refuse to register themselves as foreign agents. Tolstoi said on November 14 that media outlets that refuse "will stop working on the territory of the Russian Federation."

Shortly after the legislation’s passage, the ministry sent letters to RFE/RL’s Russian Service and to Idel.realii and Sibir.realii, Russian-language websites that focus on news from Russia's central Volga region and Siberia.

Also receiving warnings were Current Time TV, a Russian-language network run by RFE/RL in cooperation with VOA, and Crimea Realities, which focuses on news from the Ukrainian peninsula that Russia occupied and seized in 2014.

The letters did not make any specific threats, except to note that the news operations might face restrictions under the new law.

For months, Moscow has complained that RT and a state-funded news agency called Sputnik have come under increasing pressure in the United States in the past year and has vowed to respond by targeting U.S. media in Russia.

A U.S. intelligence finding in January asserted that RT and Sputnik spread disinformation as part of a Russian-government effort to influence the 2016 U.S. presidential election. Moscow has denied any such effort.

In a November 15 statement, RFE/RL said that the "situation regarding Russian media in the U.S. and U.S. media in Russia remains vastly unequal."

"RT and Sputnik distribute freely in the U.S., whereas RFE/RL has lost its broadcast affiliates in Russia due to administrative pressures, and has no access to cable," it said. "RFE/RL reporters are subject to harassment and even physical attack in Russia."

Deputy speaker Tolstoi said on November 14 that foreign-funded news organizations that refused to register as foreign agents under the proposed legislation would be barred from operating in the country. The measure would not affect Russian media that were partially financed by foreign capital, he said.

Mikhail Fedotov, head of the presidential human rights council and an author of Russia's original mass-media law, criticized the amendments, telling the RBK news service that there was no need to amend the existing mass media law and that any "symmetrical response" should be implemented by the executive branch.

"The amendments on giving foreign agent status are being adopted hastily and are badly thought out," he wrote.

U.S. officials say that the existing Russian law regarding foreign agents differs from FARA, which was passed in 1938 to counter fears of Nazi propaganda and disinformation being spread in the United States.

State Department spokeswoman Heather Nauert said last month that the need for registration under FARA "is simply triggered when an entity or an individual engages in political activity."

The developments come as ties between the United States and Russia continue to be severely strained over issues including Moscow's alleged interference in the U.S. presidential election last year and its military intervention in Ukraine.

Russian Prime Minister Dmitry Medvedev said on November 14 that "our relations are degrading day by day" and "have reached the lowest point in recent decades."